Standard point-to-point digital optical data links have a single light source which is turned on and off to send a "1" or "0" through an optical fiber to a receiver. Because of signal level variations, modal or chromatic dispersion in light transport, and other sources of loss and distortion, most of these data links use signal conditioning or encoding to ensure recovery of the original waveform. Some common requirements for such data links include a "ones density" to provide regular bit transitions, and near 50 percent duty cycle and receivers with automatic gain control to recover data over a wide variation of incoming optical signal levels. Even when such methods are used, some pulse-width distortion or bit errors can occur.
In multipoint optical communication systems, such as optical backplanes, each optical path is shared among a number of optical signal sources on a time-division multiplexed basis. In such systems, only one of many sources can typically be allowed to transmit light into the shared path at any given time. But conventional encoding and conditioning techniques typically involve continuous activity on the path, i.e., continuous transmissions by every signal source into a fiber. Consequently, no encoding of data is used in multipoint systems, and therefore no fixed duty cycle is guaranteed. Furthermore, such systems cannot normally use level-detecting receivers, since all of the possible transmitters may have different transmitted power levels, and edge detection is used instead at the receiver to recover the original transmitted waveform.
Although edge-detecting receivers have been shown to work in multipoint systems, they do have drawbacks. The overall sensitivity of an edge detection receiver is about 3 dB less than a comparable A.C.-coupled design. Because activity is bursty in nature, significant pulse-width distortion is seen in the recovered data stream. This distortion, and the use of positive feedback in the edge detection receivers, limits the maximum usable bit rate of such systems. Finally, there is no conventionally way to distinguish between transmission of a string of zeroes and no transmission of data at all. This may complicate system design.
Differential signal transmission, also known as balanced signal transmission, is a technique wherein both a pulse signal, e.g., digital binary signal, which is representative of an input signal, and a complement of the pulse signal--a signal that has the opposite signal magnitude, such that the sum of the simultaneous magnitudes of the two signals at all times is a constant--are transmitted by a transmitter to a receiver. For example, the transmitted pulse signal may alternate between magnitudes of one and zero (i.e., on and off) and its complement will respectively alternate between magnitudes of zero and one. Alternatively, the pulse signal may alternate between magnitudes of one and minus one (i.e., positive and negative) and its complement will respectively alternate between magnitudes of minus one and one. The receiver then determines the duration and value of the input signal from the instantaneous relative magnitudes of the received signal and its received complement. The advantages of differential signal transmission over other signal transmission and detection techniques are known and recognized in the art. They include enhanced immunity to signal noise, distortion, and dispersion, and enhanced sensitivity and speed of detection.
The differential signal transmission technique has been widely used to transmit electrical signals, using two electrical conductors--one for transmitting the signal itself and the other for transmitting the signal's complement. Recently, application of this technique to optical transmissions has been proposed. For example, U.S. Pat. No. 4,316,141 proposes the use of two side-by-side optical conductors, fibers, to transmit an optical signal and its complement. And U.S. Pat. No. 4,764,984 proposes transmission of an optical signal and its complement at two different wavelengths through free space, to a detector comprising two coils of optical fiber, one for receiving and conducting the signal, and the other for receiving and conducting the signal's complement.